1. Field of the Invention
The present invention relates to a document size detector, more specifically, optically detecting the size of a document placed on a platen.
2. Description of the Related Art
Existing document size detectors for image forming apparatuses include a type of detector with which a document size is detected before a document pressing plate for pressing a document against a platen is completely closed. For example, Japanese Patent Laid-Open No. 2004-258386 proposes a document size detector that functions as follows. When it is detected that the angle between a platen and a document pressing plate becomes equal to or less than a specified angle, a document illumination light source is switched on and reflected light from the document is read with a charge-coupled device (CCD). The size of the document is detected based on the result of reading by the CCD.
The document size detection is performed by utilizing the difference between an output of the CCD for the region on which a document is placed and an output of the CCD for the region on which the document is not placed. The detection is performed by assuming that a position at which outputs of the CCD differ corresponds to an edge of the document. As described above, in the document size detector disclosed in Japanese Patent Laid-Open No. 2004-258386, the document illumination light source is switched on when the angle between the platen and the document pressing plate becomes equal to or less than a specified angle. At that time, the output of the CCD for the region on which a document is not placed is less than the output of the CCD for the region on which the document is placed. This is because the amount of reflected light from the region on which the document is not placed is less than the amount of reflected light from the region on which the document is placed. This is also because only a small amount of light is reflected from the document pressing plate, since the document pressing plate is open when the document illumination light source is switched on.
The above-described document size detector starts a document size detection process when the angle between the platen and the document pressing plate becomes less than a specified angle at which disturbance light, such as light from interior illumination, negligibly enters the CCD. Thus, for a region on which a document is not placed, disturbance light negligibly enters the CCD, whereby the document size can be detected accurately without interference from disturbance light that might cause erroneous detection.
With this document size detector, a document size has been detected when the document pressing plate becomes closed. Therefore, an image forming apparatus including the document size detector is ready to start image forming as soon as the document pressing plate is closed.
However, the document size detector has a problem in that a user is subjected to glare as light emitted from the light source enters the eyes or the user when a document size is being detected, since the document illumination light source is switched on while the document pressing plate is open.
In order to reduce the amount of glare to which a user is subjected, Japanese Patent Laid-Open No. 2002-290675 discloses a document size detector in which the amount of light emitted from a light source when detecting a document size is less than the amount of light emitted from the light source when reading a document. This document size detector switches on a document illumination light source when the angle between a platen and a document cover (document pressing plate) becomes equal to or less than a specified angle, causes a CCD to receive reflected light from a document placed on a platen, and detects the size of the document based on the light received by the CCD. At this time, the light source is switched on so as to emit an amount of light less than the amount of light used for reading a document. In this way, by lighting the document illumination light source with a reduced amount of light when detecting a document size while the document cover is open, glare that a user is subjected to can be reduced.
However, when the amount of light for detecting document size is reduced as in the document size detector disclosed in Japanese Patent Laid-Open No. 2002-290675, it becomes difficult to discriminate between disturbance light and reflected light from the document, both of which are detected with a document reading sensor (hereinafter referred to as a CCD), in an environment with strong disturbance light. This may cause erroneous detection of a document size.
FIG. 13 is a graph showing the output of a CCD when a document size is erroneously detected by a document size detector in a state where, a scanning apparatus including the document size detector, is placed in an environment with strong disturbance light. Region A of the graph shows the output of the CCD resulting from reflected light from a document, and region B shows the output of the CCD resulting from disturbance light. In this case, both the output resulting from disturbance light and the output resulting from reflected light from the document may exceed a predetermined threshold for determining whether a document is present or not (hereinafter referred to as a document presence threshold). If this occurs, a controller of the document size detector may determine that the document is placed also on the region B on which the document is not actually placed, which may result in erroneous detection of the document size.
In order to prevent such erroneous detection of a document size in an environment with strong disturbance light, the amount of light for document size detection can be set so as to be sufficient to exclude the influence of disturbance light. However, this may cause a user to be subjected to glare.
As described, for a document size detector that starts a document size detection while a document pressing plate is open, there is a trade-off between a solution for preventing erroneous detection of a document size and a solution for preventing a user from being subjected to glare. A solution for both problems has not been given to date.